A number of systems have been developed to determine the composition of the earth, both at a distance near the earth's surface as well as a distance many miles below the earth's surface. For example, a shock wave is generated at the earth's surface or below the earth's surface and various sensors having utilized to sense the return wave and to process this return wave which would be an indication of the composition of the subterranean layer of the earth.
A system has been developed for determining the character of a subterranean region close to the earth's surface employing ground penetrating radar. For example, U.S. Pat. No. 3,806,795, issued to Morey repeatedly uses ground penetrating radar pulses with rise times in the order of 1 nanosecond to generate a profile chart indicating the magnitude of the reflected signals and the depths or times in which they occur. This system employs the same antenna for transmitting and receiving signals and does not utilize a synthetic aperture radar (SAR) technique, nor does it use three dimensional imaging of the reflective signals.
U.S. Pat. No. 4,325,065 and U.S. Pat. No. 4,246,580, both issued to Caputi, Jr., teach the use of a bistatic radar technique employing a spotlight SAR and stretch linear FM modulation with Fourier Transform processing. The receiver and transmitter are mounted on different aircraft which have different positions and motions during the data gathering process. Not only do these patents relate to a system in which bistatic synthetic aperture radar data and not monostatic data is important, these patents do not describe an apparatus in which the subterranean section close to the earth's surface is investigated.
U.S. Pat. No. 5,673,050, issued to Moussally et al, was developed to perform non-invasive subsurface characterization in numerous practical situations including location and characterization of buried waste, location of underground utilities and structures prior to excavation operations and the determination of subsurface geological conditions for construction, mineral extraction and environmental purposes, in depths close to the earth's surface. This system employs a stand-off, oblique angle illumination of the subsurface region using an interrupted frequency modulated continuous wave (FMCW) operation of the radar. The system would use a bandwidth of greater than one decade as well as a spotlight SAR geometry. The system would employ a near-Brewster angle illumination of the ground used to reduce losses. A radar platform would be provided in an airborne vehicle, such as a helicopter which would circumscribe the subsurface area of interest.